Prior art techniques for dynamically breaking polyphase motors, such as the three-phase induction motor, customarily introduced direct current into the motor windings by means of relay contacts in order to switch the windings from an alternating current source to a source of direct current. Several disadvantages are incurred when this technique is utilized. Since both alternating and direct currents enter the motor at the same terminals, the switching of these connections is required. In addition, inadvertent simultaneous application of vboth alternating and direct current paths, even momentarily, may result in a short circuit condition. Therefore, considerable care must be taken in switch timing such that simultaneous closure of both current paths does not occur. Finally, since a damping diode normally is not connected to the motor while it is running, care must be taken to reduce the direct current to zero before breaking the direct current path, thereby avoiding a destructive arc which may occur due to the inductance of the motor windings discharging across prematurely opened contacts.
Examples of prior art references which disclose techniques for dynamically braking a multiphase motor include U.S. Pat. No. 2,493,670 to Harvey et al which discloses a braking circuit arrangement with alternating current applied to the motor windings. Subsequent to switching the alternating current off, direct current is applied to the same windings to achieve dynamic braking.
U.S. Pat. No. 2,514,342 to Schoenbaum discloses a technique whereby rectified alternating current is applied to the motor windings in phased opposition to the current impressed thereon by the ordinary power supply circuits. The rectifier circuits, therefore, reverse the phase of the current applied to the motor and the rectifier circuits are connected in phase opposition with the corresponding power supply circuits. The rectified and alternating currents are applied to the same windings.
U.S. Pat. No. 2,858,494 to Choudhury discloses a technique for applying braking direct current while the motor windings are still being energized by the alternating current supply. In this patent, both alternating and direct currents are applied simultaneously to the same winding.
U.S. Pat. No. 3,281,632 to Ozaki discloses a technique for dynamically braking a three-phase induction motor without utilizing a separate source of direct current.